Device and method for bonding wafers

ABSTRACT

The invention relates to a device and a corresponding method for bonding wafers along their corresponding surfaces.

The invention relates to a device and a method for bonding wafers.

Various key technologies have been developed for bonding wafers or wafersurfaces and are now sufficiently known in the prior art. These include:

-   -   aligned wafer bonding    -   front-to-backside photolithography    -   customized resist coating.

The present invention primarily relates to aligned wafer bonding inwhich various methods can in turn be used. These are also describedcomprehensively in the literature. They include among others:

-   -   anodic/electrostatic bonding of silicon on glass    -   low-temperature glass frit bonding of silicon on silicon    -   direct wafer bonding (DWB) or fusion bonding

Further methods are eutectic bonding, epoxy bonding, thermo-compressionbonding and glass-on-glass bonding.

In this case, the methods described in the prior art for aligned waferbonding always consist of the following two separate and differentsteps. The two wafers arranged one above the other are first alignedwith the aid of an alignment device with an accuracy of ±1 μm, whereinthe surfaces of the two wafers which are to be bonded, lie opposite toone another. In this first step it is necessary to maintain a separationdistance between the two surfaces to be bonded. In the prior art this ismostly accomplished by spacers which are distributed on thecircumferences of the wafers, with the smallest possible surface beingcovered by these. However, in order to ensure a stable hold, at leastthree such spacers are required.

The two wafers are then placed under vacuum in a vacuum chamber afterwhich the two previously aligned wafers are brought in contact by meansof a computer control system and are joined or bonded by one of themethods described above. For contacting the spacers arranged on thecircumferences are withdrawn uniformly so that the previously alignedspacers do not slip any more.

The hitherto known methods for aligned wafer bonding consequently havetwo important disadvantages. Firstly, gaps are formed by the spacers inthe contact surface and on the other hand, during contacting bywithdrawing the spacers there is the risk that the wafers which havepreviously been positioned exactly, slip relative to one another.

It is the object of the invention to provide a method and a device withwhich disk-shaped parts, especially semiconductor disks, can be exactlyaligned and joined together in the best possible fashion.

The basic idea of the invention is to join the parts togethertemporarily at first. In this case, the parts are adjusted. The finaljoining to form a composite member then follows.

Insofar as wafer is talked about in connection with the invention, thisis representative of any type of flat component to be aligned.

Said object is solved by a device for bonding wafers to correspondingsurfaces of wafers, which has the following features:

-   -   a. a unit for applying a layer of bonding agent to at least one        of the surfaces of the wafers,    -   b. a first device for receiving and possibly aligning a first        wafer,    -   c. a second device for receiving and aligning a second wafer        with respect to a first wafer,    -   d. a control unit effecting the following process steps one        after the other:        -   i. applying a layer of bonding agent to at least one of the            corresponding surfaces by means of the unit,        -   ii. contacting the corresponding surfaces of the wafer by            means of the devices,        -   iii. aligning the wafers by aligning means along the            corresponding surfaces of the wafers in contact,        -   iv. bonding the corresponding surfaces by curing the bonding            means.

The first device can consist of a table-like support, for example, inthe fashion of a “chuck”. The table can be constructed as static orcapable of rotating. The first wafer can lie directly or indirectly onthe support.

A layer of bonding agent is applied to the second wafer and/or the firstwafer before the second wafer is brought in contact with the firstwafer. The layer can be applied to the respective wafer either as afull-surface layer or as a patterned layer.

Possible bonding agents can be adhesives, which should be selectedaccording to the substrate of the wafer and should be sufficientlyviscous in order to allow sliding of the wafers after contact of thecorresponding surfaces of the wafers. Good results are achieved, forexample, with UV-curable epoxy resin adhesives.

In this way a device which manages without contact pins is created fromwhich it follows that no gaps are formed on the contact surfaces of thecorresponding surfaces of the wafers. Furthermore, the device accordingto the invention has the advantage that a precise alignment only takesplace after the corresponding surfaces of the wafers have been broughtin contact. As a result, any slippage of the wafers after alignment bythe alignment means can be eliminated. A substantially more accuratealignment of the two wafers is achieved whereby the quality of thewafers produced using the device according to the invention increasessignificantly.

In other words: a full-surface adhesion or pattern-like adhesion takesplace along the corresponding surfaces of the parts to be joined (inthis case: wafers), without the pattern being disturbed by imprints ofpins.

The alignment means can comprise a detection device for detecting therelative position of the two wafers to be aligned with respect to oneanother, wherein the detection device transmits signals about therelative position of the wafers with respect to one another to thecontrol unit by means of a first operative connection between thecontrol unit and the detection device.

The detection device ensures the exact alignment of the wafers bydetecting the relative position of the wafers, passing this on to thecontrol unit which then aligns the wafers with respect to one another.

The detection device can be configured as contact-free, especially asoptical. For example, the relative position of the wafers with respectto one another is determined by the detection device using visibleand/or infrared light. For this purpose, integrated adjusting marks canbe applied to the wafers. In the simplest embodiment, points, symbols orareas on the two wafers can be brought into agreement for this purpose.

An even more exact alignment is achieved if the adjusting marks arescale marks which are applied to the wafers. The scale marks have apre-defined division from which the detection device can determine therelative position of the wafers with respect to one another. However,the scales can also be applied in any other form, for example, in theform of circles, one within the other, in the fashion of a target.Coloured image points with a certain arrangement are also feasible asadjusting marks.

Mechanical means which turn and/or displace the respective wafers can beused for alignment as soon as corresponding signals are obtained fromthe control unit, as defined in claim 4.

The parts to be joined can consist of glass or silicon, for example.

A relevant method for bonding two wafers on two corresponding surfacesof the wafers comprises the following steps:

-   -   a. applying bonding agents to at least one of the two        corresponding surfaces,    -   b. contacting the wafers at the corresponding surfaces,    -   c. aligning the wafers by means of sliding means along the        corresponding surfaces of the wafers in contact, by bringing        aligning means into agreement,    -   d. bonding the wafers.

In this method, the wafers can be optically aligned. A correspondingoptical measuring unit is used for this purpose. The means for alignment(adjusting) can be integrated at least partly into the parts to beadjusted. At least one adjusting mark is required on each adjustingpart. In process step B the wafers can be aligned at least roughly usingthe wafer edges. The alignment by sliding means along the correspondingsurfaces of the wafers in contact can take place by rotation and/orsliding in the X-Y direction.

With reference to further embodiments for the individual process steps,reference is made to the preceding explanations in connection with thedevice and to the following description of the figures.

Further features of the invention are obtained from the features of thedependent claims and the other application documents.

The invention is explained in detail hereinafter with reference to anexemplary embodiment. The features described therein can be importantboth individually and in any combinations for implementing theinvention. This also applies to the features which were specifiedhereinbefore to describe the device and the method. The figures in thedrawings show in detail:

FIG. 1 is a schematic view of the device according to the inventionbefore beginning the method according to the invention

FIG. 2 is a schematic sectional view along the line of intersection A-Afrom FIG. 1 which shows the process step of aligning the wafers relativeto one another after contacting.

FIG. 1 is a highly schematic diagram showing a control unit 40 whichexecutes the method according to the process steps described in theabove description in an exemplary embodiment of the present invention.

The control unit 40 here is connected in each case by a first, secondand third operative connection 41 w, 42 w and 43 w to a device 41 forreceiving and aligning the second wafer 26, to a detection device 42 todetect the position of the wafers relative to one another and to a unit43 for applying a bonding means 44.

The second device 41 for receiving and aligning the second wafer 26 isconnected to the wafer 26 via a swivelling arm 41 a to move the secondwafer 26 from the unit 43 to a first device 20 for receiving the firstwafer 22. The spatial arrangement of the devices with respect to oneanother is merely exemplary.

Grippers at the ends of the arm 41 a, which are not shown, hold thesecond wafer 26 and are known to the person skilled in the art. It isadvantageous to use contactless Bernoulli grippers.

Using the unit 43, an adhesive is applied to the second wafer 26 and/orthe first wafer 22 by known methods such as spraying on, centrifugingon, printing on or other methods. The adhesive layer can be applied overthe entire surface or structured, for example, by screen printing.

As soon as the bonding means (the adhesive) has been applied to one orboth wafers, the corresponding surfaces of the wafers are brought incontact, by swivelling the second wafer 26 with the swivelling arm,controlled by the control unit 42, over the first wafer 22 and loweringit onto this.

As soon as the two wafers 22 and 26 are in contact, the detection device42 detects the position of the wafers 22 and 26 relative to one anotherby means of at least two high-resolution objectives 42 o which areconnected via arms 42 a to a housing of the detection device 42. Forthis purpose two adjusting marks 23 and 27 are integrated in each of thetwo wafers 22 and 26. The adjusting marks 23 and 27 can be applied tothe wafers 22 and 26 or associated with these. Detection can also takeplace using one objective 42 o. The accuracy is substantially higherhowever, as a result of the shorter possible distance from the adjustingmarks 23 and 27, when one objective per adjusting mark 23, 27 is used.It is also possible to use merely one objective 42 o and a relevantadjusting mark on each wafer 22 and 26 for detection by aligning merelyusing the configuration of the adjusting mark. Using more than twoadjusting marks only insignificantly increases the accuracy which is whythis can be dispensed with for cost reasons.

The objectives 42 o are swivellably attached to the housing of thedetection device 42 via arms 42 a so that the objectives 42 o can bebrought out of the area of action of the arm 41 a and the wafer 26 whenthe two wafers 22 and 26 are in contact.

The detection device 42 then transmits the signals to determine therelative position of the two wafers 22 and 26 relative to one another tothe control unit 40 which brings about a sliding of the second wafer 26with the aid of the device 42 according to the position of the wafers22, 26 with respect to one another. Sliding of the two wafers 22 and 26in contact is possible by selecting a suitable adhesive 44 which issufficiently viscous to allow sliding by introducing a force parallel tothe wafer surface. The viscosity can additionally be influenced bycontrolling the temperature. For alignment of the wafers 22, 26 thesecan be slid and/or turned.

The adjusting marks 23 and 27 are constructed as corresponding scalelines on the two wafers so that an exact adjustment with a deviation of<=500 nm is achieved with a division of the scale lines of, for example,500 nm within the optical resolution of the objective 42 o.

As soon as the adjustment has ended, the curing of the bonding agent isbrought about by the control unit, this being achieved for example bymeans of UV light or increasing the temperature. A device for curing thebonding agent is not shown here.

Alternatively, the alignment of the two wafers 22 and 26 can also oradditionally be achieved by sliding and/or rotating the device 20 toreceive the first wafer 22.

The device 41 is at the same time used here as a sliding means for thealignment step according to the present invention. Nevertheless, it isalso feasible to use an additional device as a sliding means which isonly responsible for the exact positioning of the wafers with respect toone another. As a result, the major swivelling of the second wafer 26can be carried out using a device which is substantially more inaccurateand therefore also cheaper.

The entire device can also be arranged in a vacuum chamber in whichfurther usual process steps such as a pre-cleaning step or flushing withinert gas can also be integrated.

Said process steps are instigated and monitored by a control unit 40which is connected to the individual parts of the device by controltechnology.

The method described here accelerates the bonding of wafers since theprocess step of inserting spacers between the wafers and removing theseagain uniformly is eliminated. The precision of the alignment is furtherincreased because of the shorter distance of the adjusting marks fromone another compared to the devices from the prior art. A furtheradvantage compared with the prior art is that it is possible to haveadhesion over the entire surface.

1. A device for bonding wafers (22, 26) on corresponding surfaces (22 o,26 o) of the wafers (22, 26) having the following features: a. a unit(43) for applying a layer of bonding agent (44) to at least one of thesurfaces (22 o, 26 o) of the wafers (22, 26), b. a first device (20) forreceiving and possibly aligning a first wafer (22), c. a second device(41) for receiving and aligning a second wafer (26) with respect to afirst wafer (22), d. a control unit (40) effecting the following processsteps one after the other: i. applying a layer of bonding agent (44) toat least one of the corresponding surfaces (22 o, 26 o) by means of theunit (43), ii. contacting the corresponding surfaces (22 o, 26 o) of thewafer (22, 26) by means of the devices (20, 41), iii. aligning thewafers (22, 26) by aligning means along the corresponding surfaces (22o, 26 o) of the wafers (22, 26) in contact, iv. bonding thecorresponding surfaces (22 o, 26 o) by curing the bonding means (44). 2.The device according to claim 1, whose aligning means comprises adetection device (42) for detecting the relative position of the twowafers to be aligned (22, 26) with respect to one another, wherein thedetection device (42) transmits signals about the relative position ofthe wafers (22, 26) with respect to one another to the control unit (40)by means of a first operative connection (42 w) between the control unit(40) and the detection device (42).
 3. The device according to claim 1whose detection device (42) is constructed as contact-free, especiallyas optical.
 4. The device according to claim 1, whose alignment meanscomprises sliding means (41) which can be controlled by the control unit(40) via a second operative connection (41 w) and aligns the wafers (22,26) relative to one another in a pre-defined relative position by meansof the control signals transmitted by the control unit (40).
 5. Thedevice according to claim 4 wherein the sliding means (41) is the seconddevice (41).
 6. The device according to claim 2, wherein the detectiondevice (42) determines the relative position of the wafers with respectto one another (22, 26) by means of visible and/or infrared light byadjusting marks (23, 27) attached or integrated on the wafers (22, 26).7. The device according to claim 1, wherein two adjusting marks areattached or integrated on each wafer (22, 26) wherein the adjustingmarks (23, 27) are preferably arranged at a distance from one another.8. The device according to claim 7, wherein the adjusting marks (23, 27)are scale lines which are applied to the wafers (22, 26).
 9. The deviceaccording to claim 1, wherein the second wafer (26) consists of glassand the first wafer (22) consists of silicon.
 10. A method for bondingtwo wafers on corresponding surfaces of the wafers comprising thefollowing steps: a. applying bonding agents to at least one of the twocorresponding surfaces, b. contacting the wafers at the correspondingsurfaces, c. aligning the wafers by means of sliding means along thecorresponding surfaces of the wafers in contact, by bringing aligningmeans into agreement, d. bonding the wafers.
 11. The method according toclaim 10, wherein the wafers are optically aligned.
 12. The methodaccording to claim 10, wherein the alignment means are at least partlyintegrated in the wafer.
 13. The method according to claim 10, whereinthe wafers are aligned by at least one adjusting mark of the alignmentmeans integrated in each wafer.
 14. The method according to claim 10,wherein preferably optical detection means of the alignment means areused to detect the alignment.
 15. The method according to claim 10,wherein during the contacting in step b the wafers are roughly alignedusing the wafer edges.